Electrophilic Aromatic Substitution

1. The arenium ion (sigma complex)

In the first step of EAS, the aromatic π system uses two electrons to form a new σ bond to the electrophile E⁺. The resulting intermediate is a carbocation whose positive charge is delocalized over three ring carbons (ortho, para, and the substituted carbon) by resonance. This arenium ion (also called a sigma complex) has lost aromaticity, which is why it is a high-energy intermediate and why the second step is fast.

2. Deprotonation restores aromaticity

A weak base (often the conjugate base of the acid catalyst, e.g., FeBr₄^-) removes the proton from the substituted carbon. The electron pair of that C–H bond returns to the π system, regenerating the aromatic sextet and giving the substituted product. Aromaticity is the thermodynamic driving force of the whole reaction.

3. The five classic EAS reactions

Each reaction is a way to generate a particular electrophile: halogenation (X₂ + Lewis acid → X⁺); nitration (HNO₃/H₂SO₄ → NO₂⁺, the nitronium ion); sulfonation (SO₃/H₂SO₄ → SO₃/HSO₃⁺); Friedel-Crafts alkylation (alkyl halide + AlCl₃ → carbocation); and Friedel-Crafts acylation (acyl halide + AlCl₃ → acylium ion RCO⁺).

How it works

1. A strong electrophile is generated that is electron-poor enough to be attacked by the electron-rich aromatic ring.
2. The ring donates a π pair to form a C–E bond, transiently sacrificing aromaticity to give the arenium ion.
3. Rapid loss of a proton returns two electrons to the π system, restoring the aromatic sextet.
4. The specific electrophile (and thus product) is chosen by reagent: halogenation (X), nitration (NO₂), sulfonation (SO₃H), alkylation (R), acylation (RCO).

Common confusions

Memory aids

Remember the steps as "GEAR" — Generate the electrophile, Electrons Attack (arenium ion), Remove a proton (Aromaticity restored). For reagents, recall "H-N-S-A-A" — Halogenation, Nitration, Sulfonation, Alkylation, Acylation — five flavors of the same mechanism.

Quick review

Topic Recap

EAS is the signature reaction of aromatic compounds: a strong electrophile attacks the ring to give a resonance-stabilized arenium ion, and deprotonation restores aromaticity. Halogenation, nitration, sulfonation, and the two Friedel-Crafts reactions are all variants of this single mechanism. Alkylation's rearrangement/polyalkylation problems are solved by acylation plus Clemmensen reduction. Recognizing which electrophile each reagent generates is the key to predicting products.

Knowledge Check

1. Draw (in words) the arenium ion formed when benzene attacks Br⁺ and indicate where the positive charge is delocalized.
2. What is the electrophile in nitration, and how is it generated?
3. Why does Friedel-Crafts alkylation often give a rearranged product?
4. How can one cleanly install an n-propyl group on benzene without rearrangement?
5. Why does benzene undergo substitution rather than addition with Br₂?

Answers and Rationales

1. The arenium ion has Br bonded to one carbon, with positive charge delocalized over the ortho and para carbons (three resonance forms). Aromaticity is lost (only four π electrons remain delocalized over five carbons).
2. The nitronium ion NO₂⁺, generated when sulfuric acid protonates nitric acid and water leaves. It is a very strong electrophile.
3. The initially formed primary carbocation rearranges (hydride or alkyl shift) to a more stable secondary/tertiary carbocation before attacking the ring, giving a rearranged alkylbenzene.
4. Friedel-Crafts acylation with propanoyl chloride/AlCl₃, followed by Clemmensen reduction (Zn(Hg)/HCl). The acylium ion does not rearrange, and reduction converts C=O to CH₂.
5. Addition would destroy aromaticity and raise the energy of the system; substitution restores the aromatic sextet, so the aromatic product is far more stable and the reaction path to it is favored.